Everyone knows that links increase sites PR and hence number of visitors, there are numerous ways to get those links organically (like exchanging links with relative resources, registering in directories, etc.), but isn't it simpler just to buy links? How ethical it is and how effective could be?

Well, if you think that link is like a banner, than is should be a normal practice to buy and sell links. From the other side the result from this deal can be only temporary, as actually you are buying link for some time - more like renting it.
The good side is - you can have quick result for a short time!

So, how much it's reasonable to pay for such a link?
PR - 4 $ 15 to $ 70
PR - 5 $ 40 to $ 110;
PR - 6 $ 150 to $ 250
PR - 7+ $ 450+
(for a month)

Still, every site owner should remember that links are not just for Googles PR, they are for potential visitors, so it's important to buy links from relative resources.


(materials translated from http://www.whiteseo.ru/content/view/183/6/)

Buying links will initially help but soon will be caught by google. And once caught, your site can be penalised by the google. To get a good PR just go for the natural links and not for the paid links. It takes time but will improve sites's rankings in SE's as well.

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Search Engine Tips

Personally, I think it is a grey area. Technically, someone would have to report your site and the sites the links were purchased. Now your average Joe won't have a clue and if so, he wouldn't bother reporting you to Google, however there are now agencies who are offering "negative" SEO where you can penalise your competiors in the SERP's.

See also:

The Saboteurs Of Search

and most recently:

The Great Google Directory Ban Of Sept. 2007

hello
if you make your site seo friendly with good content and it's in serp so you don't need to buy link from someone .because then people would like to link you free .